The present invention relates to a deproteinizing agent used to obtain a deproteinized natural rubber latex which is stable and exhibits cationic properties, wherein a protein in a natural rubber is removed, and a method of preparing a cationic deproteinized natural rubber latex using the same.
Although a natural rubber latex contains a protein as a non-rubber component other than the rubber component, the kind and amount of the protein vary depending on the producing district and producing season, thereby causing problems such as scatter in quality and vulcanization characteristics of a final natural rubber product as well as deterioration of qualities of the natural rubber product, for example, mechanical characteristics such as creep characteristics and aging resistance, and electrical characteristics such as insulating properties.
Also there has recently been reported the case where a strong I type allergic symptom is caused by a protein in a natural rubber when using a natural rubber product.
To solve these problems, a method of concentrating a natural rubber latex by washing with water and a method of concentrating the latex by washing in the presence of a surfactant are disclosed, however, insufficient deproteinization degree was attained by these methods.
Also a trial of solving these problems has been made by a method of efficiently removing a protein from a natural rubber latex disclosed in Japanese Published Unexamined Patent (Kokai Tokkyo Koho Hei) Nos. 6-56904, 6-56905 and 6-56906. Such a method is characterized in that a proteolytic enzyme (protease) and an anionic or nonionic surfactant are added to a natural rubber latex, thereby to decompose a protein, and then a creamy rubber component is separated by a purification treatment such as centrifugation to remove the protein.
However, the surfactant used in the method of the publications described above is anionic or nonionic and rubber particles in the deproteinized natural rubber latex are negatively charged or in the state free from polarity.
Accordingly, it was not preferable to use, as a raw material for an adhesive, such a rubber latex in the adherend, which is negatively charged originally, such as fibers and paper products.
On the other hand, a cationic natural rubber latex incorporated with cetyltrimethylammonium bromide (CTAB) as a cationic surfactant is described in pages 94 to 95 of "Natural Rubber Science and Technology" (edited by A. D. Roberts, published by Oxford University Publication, 1988). As described in such a reference document, when a sufficient amount of the cationic surfactant is added to the natural rubber, a latex comprising rubber particles having cations can be obtained. The latex is made cationic in such way, thereby making it possible to improve the close adhesion with the fibers or papers which are negatively charged when treated in water.
However, deproteinization can not be attained as a matter of course only by adding the cationic surfactant and the allergic reaction occurs, thereby making it impossible to provide a natural rubber latex which is stable to the human body.
Furthermore, there is such a problem that the viscosity increases with a lapse of time to form a solidified substance in case the cationic surfactant is merely added in the natural rubber latex, thereby making it impossible to obtain a stable latex.